Custom data annotation with RIa Services

I have a Silverlight application and I'm using Entity Framework and Ria Services.

I need to create a custom attribute that I will use to recognize some properties.

Ex. :

public class Person
{
    [IsSpecialProperty]
    public string PersonProperty { get; set; }
}

public class IsSpecialPropertyAttribute : Attribute
{

}

I named the class file "IsSpecialPropertyAttribute.shared.cs" to access it from the client.

And from the client side, I use the method :

var attributes = (IsSpecialPropertyAttribute[])Attribute.GetCustomAttributes(memberInfo, typeof(IsSpecialPropertyAttribute));

This method is working for system attributes but not with my custom attribute...

Any idea why ?